Allow RenderImGui to work without a vsg::Window

[geefr]

Fairly straightforward at the vsgImGui level - New constructor added and _init methods shuffled around to allow construction without the vsg::Window handle.

One extra fix to ensure ImGuiIO::DisplaySize is initialised to something - I assume this is normally set when handling input or otherwise rendering to the window, but when rendering to texture it doesn't get initialised, and can assert inside ImGui.

Tested by a quick example setup, wasn't planning on merging it to vsgExamples but could if needed:

• https://github.com/geefr/vsgExamples/blob/vsgimgui-render-to-texture/examples/ui/vsgimgui_rendertotexture_example/vsgimgui_rendertotexture_example.cpp

[robertosfield]

Thanks @geefr, changes look sensible now merged with master.